The invention relates to vapor generators and, more particularly, to a downhole vapor generator utilizing the combustion of air and fuel to radiantly and convectantly heat water for the creation of steam and the pressurized injection thereof into adjacent down-hole hydrocarbon formations.
Many forms of stimulation processes have been employed for increasing productivity of hydrocarbon deposits such as oil and gas wells. The devices utilized in such stimulation processes generally include the generation of both heat and pressure to lower the viscosity of adjacent petroleum, eliminate deposits of materials such as paraffin and impart flow to an adjacent production well. Many processes utilizing such concepts have been employed and/or taught in the prior art. One apparatus is shown and described in U.S. Pat. No. 3,315,745 which discloses a bottom hole burner for introducing heat directly into a downhole formation. The burner comprises a combustion chamber for the mixing of fuel and air and the in situ combustion thereof within the well. An ignition device is provided in the upper end of the combustion chamber. The combustion creates a high pressure level and hot gases are emitted as long as combustion is maintained. These devices are useful for sustaining in situ combustion from oil in the formation surrounding the well.
Another down-hole heating technique is set forth and described in U.S Pat. No. 3,980,137, issued Sept. 14, 1976 to William W. Gray, which discloses a steam generation and injection system. Oil and gas well production has been shown to be increased by pumping pressurized steam directly into the well as compared to the in situ down-hole combustion technique set forth above. The injection of steam not only heats the formation but also facilitates the elimination of deposits of materials such as paraffin as well as dissolving obstructions that impede the flow of petroleum products in such formations into producing wells. It has been shown that an increase in reservoir temperature from 81.degree.. Fahrenheit to 200.degree. Fahrenheit results in a 27 fold decrease in crude coil viscosity. A decrease in the viscosity affords an increase in the free flowability of what otherwise would be termed as "frozen" oil.
Steam injection systems of the prior art have incorporated fuel-air mixtures delivered to combustion chambers of various designs disposed in downhole configurations. Steam is generally generated from water delivered directly into the combustion chamber where it is converted into vapor. The temperature and pressure of the vapor passing from the combustion chamber is then controlled by adjusting the flow rate of the fuel-air mixture as well as the flow rate of coolant, or water, delivered thereto. Heat transfer to feed water in such combustion chambers is effected primarily by conduction rather than through radiation heating from the flame. Such combustion is often stoichiometric and generally sustained by a mixture of hydrogen and oxygen. Since hydrogen combustion creates very little radiant heat, such systems prevent over-heating of the adjacent well casing which can deleteriously occur with escape of radiant energy from less advanced down-hole combustion apparatus.
The general problems of prior art methods and apparatus for downhole burners have included the over-heating of adjacent well casing, inefficient heat dissipation, operation cost, efficiency and reliability. It would be an advantage to use fuels less expensive than hydrogen due to the enormous related expense of secondary recovery operations. However, an efficient and reliable system must be provided for down-hole use.
The creation of steam by vapor generators encompasses a wide range of prior art technology. For example, early torpedo designs have utilized vapor generators for propulsion. One such structure is described in British Pat. No. 140,156 accepted Mar. 23, 1920. The vapor generator set forth in the British reference utilizes steam and the products of combustion for creating kinetic energy to drive the torpedo. The fuel is burned under suitable pressure in a combustion chamber. At one end of the chamber the burners are situated while the other end the chamber is open to mixing. Water is supplied to an annular space surrounding the combustion chamber. Water flowing through the annular space cools the combustion chamber walls while being heated. The flame from the burner fills the combustion chamber and the flames strike the water egressing from the annular space converting the preheated water into steam.
The aforesaid concept has been incorporated into several varieties of downhole steam injector structures. A more recent construction is set forth in U.S. Pat. No. 4,243,098 issued Jan. 6, 1981 to Thomas Meeks, et al. This reference teaches the use of a closed tubular flow path within an annular heat exchanger. The tubular array carries the water to be heated along the wall of the combustion chamber of the vapor generator. This closed system approach provides an alternative method to the open flow technique set forth in the Gray patent, while utilizing a similar direct flame engagement process.
In a downhole well bore application, certain aspects of vapor generation are critical and must be closely controlled. For example, gases can become trapped in the coolant, or feed water. These gases which may come from a number of sources, can bubble out causing vapor-lock in a closed system and/or separate the coolant from chamber walls in an open system. Such conditions can lead to serious over-heating of the heat exchanger. For example, the older torpedo concept described above is effective in the generation of steam from closed, annular heating region about a combustion chamber, but there is ample room to dissipate excess heat. The particular water, chemical, mineral compositions found in down-hole operations which contribute to out gassing thus necessitate improvements to certain of the aforesaid steam generator designs of the prior art.
It would be an advantage therefore to provide a downhole vapor generator having improved features of out-gas control, maximization of heat generation and minimal heat dissemination into the adjacent well bore casing. The vapor generator of the present invention provides such a method and apparatus by incorporating a combination closed-open flow system in a concentrically aligned combustion and feed system having an annular heat exchanger about a segregated, centralized combustion zone. The water within the annular heat exchanger flows through a semiopen tubal array and is heated through a segregated thermal radiation zone rather than brought into direct contact with the flame and prior to mixing with the products of combustion in the combustion chamber, for egressing into the adjacent hydrocarbon formation.